In the art of pressure fluid actuated reciprocating piston-hammer percussion rock drills and similar percussion tools, it is known to provide the general configuration of the tool to include a sliding sleeve type valve for distributing pressure fluid to effect reciprocation of a fluid actuated piston-hammer. There are many applications of these types of drills including, for example, drilling holes having a diameter ranging from about 4 centimeters to about 30 centimeters.
Examples of such drills are generally disclosed and claimed in U.S. Pat. No. 5,680,904, issued Oct. 28, 1997. The percussion rock drill disclosed in the '904 patent includes opposed sleeve type valves disposed on opposite reduced diameter end portions of the reciprocating piston-hammer, respectively, for movement with the piston-hammer and for movement relative to the piston-hammer to distribute pressure fluid to opposite sides of the piston-hammer to effect reciprocation of same. Another advantageous design of a fluid actuated percussion rock drill is disclosed and claimed in U.S. Pat. No. 4,828,048 to James R. Mayer and William N. Patterson. The drill described and claimed in the '048 patent utilizes a single sleeve type distributing valve disposed at the fluid inlet end of the drill cylinder.
In such drills the shank may be moved forward, out of its power position, when drilling is no longer required. Such is the situation when the drill is being pulled out of the hole. During this time, however, the sliding sleeve type valve permits the high pressure fluid to continuously drive the piston-hammer. Accordingly, unless impeded, a front landing of the piston-hammer will strike the forward moved shank. Moreover, as the shank is moved forward there is additional length in which the piston-hammer may gain speed. Thus, in some cases the front landing of the piston-hammer strikes the forward moved shank with a force greater than that experienced during operational drilling. Such excessive impact causes components such as the shank to wear unnecessarily. Accordingly, it is desirable to reduce or eliminate such excessive impact. Prior methods of doing so having included the use of shock absorbers, cushions and/or springs to absorb the energy of the piston-hammer. These devices and methods, however, wear themselves and require replacement.
Therefore, what is needed is an improved internal dampening system that is wear resistant.